With the advent of the LHC, high multiplicity pp and pPb collisions show unexpected phenomena which
have never been observed before in such small systems.
The observation of a long range rapidity ridge in the measurement
of two-particle angular correlation in heavy-ion collisions is no surprise to us and this can be well explained by
hydrodynamical collective flow of a strongly interacting and expanding medium [1]}. However, the appearance of similar structures
in a high multiplicity pp and pPb collisions has drawn a lot of attention and prompted studies to understand the cause of such behaviour
in small systems. In particular, the discovery of the ridge by CMS collaboration [2]} in high multiplicity pp collisions is
one of such intriguing results observed in small systems [3]}.
Long-range, near-side angular correlations in particle production emerged in pp and subsequently in pPb collisions
paved the way for a systematic investigation of the existence of the collective phenomena.
Much information can also be gained by focusing on collective properties of each event, such as multi-particle correlations, or event-by-event fluctuations of such quantities.
We observe signatures traditionally attributed to a collective behaviour not only in PbPb collisions but also in small systems. Since then, a wealth of new,
unexpected phenomena has been observed with striking similarities to heavy-ion observations.
